What ∯ means
Use it when the integration domain wraps around a solid.
The integral sign ∫ grew from Leibniz's long S for Latin summa (“sum”) in 1675. Surface integrals developed in mechanics and physics to add up a field's contributions across a surface and relate that total to the volume enclosed. Lagrange treated them in 1811.
Formal notation for equations, calculus, and physics.
The circle marks closure.
Don't confuse it with
2Questions people ask
1Why does ∯ show as a box on some devices?
Because that device's fonts have no glyph for it. The character is still correct and still copies correctly — the box is a missing glyph, not a broken character.
How to type ∯
Windows (Word)
Alt+XType 222F, then Alt+X to get ∯.
Windows
Win+Rrun charmap for the full table, or use the emoji picker's Ω symbols tab (Win + .).
macOS
Control+Command+SpaceOpen the character viewer and search “surface integral”.
Linux
Ctrl+Shift+UType 222f, then Enter or Space.
HTML
∯CSS
\222Fhow to type oiint
In Microsoft Word's equation editor, type \oiint and press Space.
Codes and encoding1 code points
HTML decimal
∯
HTML hex
∯
CSS content
content: "\222F";
Each escape is closed by the next backslash; a trailing space is needed before a literal hex digit.
URL encoded
%E2%88%AF
1
On screen
1
Code points
1
JS .length
3
UTF-8 bytes
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.